Three U.S. patents describe macro-scale vortex structures aimed primarily at slag recovery: U.S. Pat. No. 4,011,068 (Mar. 8, 1977), U.S. Pat. No. 4,140,632 (Feb. 20, 1979) and U.S. Pat. No. 4,146,468 (Mar. 27, 1979). All three patents are expired. However, those devices are not miniaturized and include an inner porous vertical wall. Also, these prior systems rely on gravity for sedimentation and can therefore only separate out particles with sedimentation times shorter than the residence time of the particles within the structures. This restricts minimum particles sizes to 70-150 um. Daniel Chiu has also published a brief communication: “High Radial Acceleration in MicroVortices,” Nature, Vol. 425, September 2003 which describes driving a shear fluid to couple into one corner of a micron-scale pancake structure to create high accelerations.
Particle separation and concentration is an important requirement especially in biological and chemical processes for both macro-scale and miniaturized lab-on-chip applications. Some of the methods employed today are mechanical sieving and sedimentation which are usually reserved for separation of large particles. Techniques such as hydrodynamic chromatography, size exclusion chromatography and electrophoresis allow separation of smaller particles.
In addition, U.S. application Ser. No. 11/606,458, entitled “Serpentine Structures for Continuous Flow Particle Separations,” filed Nov. 30, 2006 by Lean, et al. and U.S. application Ser. No. 11/606,460, entitled “Particle Separation and Concentration System”, filed Nov. 30, 2006 by Lean et al. are incorporated herein by reference in their entirety and relate to particle flow separation systems.
These applications make advantageous use of the phenomenon of a radial or transverse pressure gradient to affect efficient separations. However, in a vortex structure, much higher transverse pressure gradients may be realized and used to separate sub-micron particles.
Large scale water purification and mining/mineral recovery applications require large volume, high throughput, and rapid processing capabilities. Current water purification methods require sand beds and even membrane filters depending on the desired water quality. Mineral processing systems use a spiral concentrator design where a helical trough allows heavy minerals to sediment near the center while centrifugal force pushes lighter particles outward where they are transported away. The tray has a sloped cross-section which is deeper near the axis of the helix.
Another important application is bio-defense, where the challenge is to determine and detect biological threats in the water supply. The United States Department of Defense (DoD) has set standards for expected limit of detection (LOD) for a list of potential agents. In particular, the Tri-Service Standard for anthrax spores is 100 cfus/L, which poses a significant challenge in logistics, time, and concentration factor. Neglecting all losses, at least 1000 L of water must be screened with a concentration factor of 106 for a typical detector sensitivity of 105 cfus/mL. The most popular method for screening large volumes of water is tangential flow filtration (TFF) with low molecular weight cut-off (MWCO) membranes (typically 30 KDa). Commercial providers of these pre-engineered systems include Pall Filtration, Millipore and US Filter. The biggest challenge to this method and to all these vendors is the low yield and laborious recovery of captured pathogens from these membranes.